In situ quantification of the active acid sites of H6P2W18O62·nH 2O heteropoly-acid through chemisorption and temperature programmed surface reaction of isopropanol

Abstract

Isopropanol chemisorption and temperature programmed surface reaction analyses were used to determine the nature, amount and acid strength of the active sites of bulk WO3, monolayer supported tungsten oxide over titania and phospho-tungstic Wells-Dawson and Keggin type heteropoly-acids. The temperature conditions were adjusted to cover the surface of the catalysts with a stable monolayer of intermediate chemisorbed isopropoxy species avoiding the adsorption of molecular isopropanol (non reactive) or surface reaction during the chemisorption process. The chemisorption of isopropanol at 40°C led to the coverage of the catalysts with a monolayer of adsorbed isopropoxy species and avoided surface reaction. Heteropoly-compounds were highly active towards isopropanol dehydration above that temperature. The number of available sites for isopropanol adsorption-reaction of Keggin and Wells-Dawson heteropoly-acids was considerably higher than bulk WO3 and monolayer supported tungsten oxide species. The temperature of surface decomposition of adsorbed isopropoxy species towards propylene was considerably lower on the heteropoly-acids rather than the other tungsten oxide based catalysts. The degree of hydration of the heteropoly-anions greatly modified the amount of accessible sites for isopropanol adsorption and further dehydration.